This invention relates to web handling systems, one example of which comprises magnetic tape drives, and, more particularly, to web handling systems in which the web is maintained under tension during high speed motion.
Web handling systems, such as magnetic tape drives, respectively wind and unwind a web with respect to supply and take-up spools to transport the web along a web travel path between the spools. The web is transported under tension so as to avoid any slack and to avoid the possibility of jumping out of the web travel path. In the field of magnetic tape drives, the web travel path is termed the xe2x80x9ctape pathxe2x80x9d, and tension is required to insure that the magnetic tape remains in close proximity to a read/write tape head positioned in the tape path as the magnetic tape is transported across the tape head, allowing the tape head to read and/or write data with respect to the magnetic tape.
High speed magnetic tape drives have buffered or decoupled the magnetic tape along the tape path, for example, with a vacuum column buffer, which when small in size may be termed a xe2x80x9cpucker pocketxe2x80x9d, in order to compensate for variations in tension as the spools are rotated. In a long tape path having a fluid pressure bearing or bearings, the fluid bearings may absorb and buffer small variations in tension. Other magnetic tape drives and other web handling systems may employ mechanical tension buffers such as spring-loaded idler wheels to absorb and buffer variations in tension.
Variations in tension come from any or all of several sources. Major variations in tension may come from variations in speeds of the tape at one or both spools. U.S. Pat. No. 4,126,817, Luzio, operates the supply spool of a document positioning web of a reproducing machine as a derivative of the operation of the take-up spool to maintain a relatively constant tension on the web as it is accelerated or decelerated.
In magnetic tape drives, it has been recognized that, if the rotational velocity of tape reels are unchanged, the relative velocities of the tape at each reel varies as the tape is wound onto one reel and unwound from the other reel. Coassigned U.S. Pat. No. 4,015,799, Koski et al., adjusts the tape reel drive motor drive currents to adjust the rotational velocities of the reels to drive the tape at a constant velocity while maintaining tension on the tape by a xe2x80x9csum of torquesxe2x80x9d model. Koski et al. analyze reel and tape speed tachometers to determine the torques required for each drive motor to keep the tape under tension and in motion at the constant velocity. Coassigned U.S. Pat. No. 4,125,881, Eige et al, describes the calculations in greater detail, and employs a tape tension measurement for fine adjustments, and U.S. Pat. No. 5,576,905, Garcia et al., is a bi-directional version. U.S. Pat. No. 4,531,166, Anderson, provides an adaptive algorithm which uses initialization to determine if parameters have changed over time.
As the web handling systems, such as magnetic tape drives, become smaller, it becomes desirable to have a more direct path between the supply and take-up spools. A more direct path is, however, more sensitive to high frequency tension variations which occur, for example, during a single revolution of a spool, in that a tension variation may alter the speed of the magnetic tape at a tape head in a magnetic tape drive. It is also desirable to provide longer lengths of magnetic tape on a reel of a cartridge, and thinner tape is a means for providing longer tape without changing the size of the reel or cartridge. Hence, the magnetic tape itself becomes more sensitive to such high frequency tension variations, and may tend to distort. Further, the magnetic tape may be more flexible and be subject to sideways motion as the result of such high frequency tension variations, with the result that the recording tracks of the magnetic tape may move laterally in excess of the capability of the read/write tape head to adjust.
For example, U.S. Pat. No. 5,909,335, Hardeng, provides a motor control system for providing a speed control for magnetic tape, but also provides xe2x80x9cflutter rollersxe2x80x9d to xe2x80x9cabsorb a high frequency speed variation, known as flutterxe2x80x9d. The IBM 3590 Magstar employs a small xe2x80x9cpucker pocketxe2x80x9d to absorb tension variation, including high frequency tension variation.
High frequency tension variation also comes from any or all of several sources. One example comprises a molded spool, such as a magnetic tape reel hub in a magnetic tape cartridge, in which the spool itself is round, but which is off-center with respect to a drive engagement means for the reel at which a drive mechanism of a tape drive is inserted. Another example comprises a single reel magnetic tape cartridge in which the magnetic tape is wound on a supply reel in the cartridge, and the lead end of the tape comprises a leader block or threader which is inserted into a slot on a take-up reel in the tape drive. The leader block is not necessarily the exact size of the slot on the take-up reel, and may be smaller or larger than the slot, such that the magnetic tape extends into the slot to provide an effectively smaller radius, or is extended above the hub to provide an effectively larger radius. In a further example, a web handling spool may be manufactured such that it is distorted from a perfectly round shape. In a still further example, a driving shaft for a web handling spool may be mounted off-center with respect to the spool, such that the spool forms an eccentric.
Tension buffers tend to be expensive and to consume valuable space. There is a desire, for example, to reduce the size of magnetic tape drives as much as possible while maintaining the largest possible length of magnetic tape in a cartridge, the large length of tape allowing storage of a greater amount of data on the magnetic tape. These two goals are best met with a short, direct tape path between the supply and take-up reels, as discussed above, with a read and/or write head positioned so as to be intermediate the supply and take-up reels along the tape path, and not allowing room for a tension buffer along the tape path. A further goal is the reduction of the cost of the tape drive, and installation of a small vacuum xe2x80x9cpucker pocketxe2x80x9d is an expensive proposition.
It is an object of the present invention to reduce high frequency tension variation in web handling systems, such as magnetic tape drives.
A web handling system, such as a magnetic tape drive, comprises, in one embodiment, respectively, a first spool, a first drive motor for rotating the first spool, and a first tachometer for indicating the rotary position of the first spool, the first tachometer having an index indication; and a second spool, a second drive motor for rotating the second spool, and a second tachometer. In a magnetic tape drive, one or both spools (or reels) may be part of a removable cartridge, and not part of a magnetic tape drive per se.
The web handling system respectively winds and unwinds a web with respect to the spools to transport the web between the spools under tension.
In a magnetic tape drive, at least one read and/or write head is positioned so as to be intermediate the first and the second spools along a tape path, for reading and/or writing data to a magnetic tape as it is transported along the tape path between the first and the second spools when being respectively wound and unwound with respect to the first and the second spools.
A control system is coupled to the first drive motor, the first tachometer, the second drive motor and the second tachometer, and operates the first and the second drive motors for respectively winding and unwinding a magnetic tape with respect to the spools to transport the magnetic tape between the spools under tension.
In one embodiment, the control system compensates for web tension variation caused by the first spool, employing the following method:
(A) determining, from the first tachometer, the rotational frequency of the first spool;
(B) determining, from the second tachometer, variation in rotational velocity at the second spool which occurs at the frequency of the step (A) determined rotational frequency of the first spool;
(C) determining, from the first tachometer index indication, the rotational position at the first spool corresponding to the step (B) determined variation in rotational velocity at the second spool;
(D) calculating a drive motor profile for operating the first drive motor which tends to cancel the step (B) determined variation in rotational velocity at the second spool; and
(E) superimposing the step (D) drive motor profile on the first drive motor at the step (C) determined rotational position of the first spool.
Thus, the present invention compensates for eccentricity in the first spool by calculating and superimposing a drive profile on the drive motor for the first spool. The drive profile is determined based on the variation in rotational velocity at the second spool caused by eccentricity in the first spool, and is known to be caused by the first spool since the variation is at the rotational frequency of the first spool.
The step (B) variation in rotational velocity determination may comprise employing a discrete Fourier transform (DFT) at the frequency of the step (A) determined rotational frequency, for determining the amplitude and phase of the determined variation.
The step (B) variation in rotational velocity determination may further comprise employing a linear resonating filter at the frequency of the step (A) determined frequency, for determining the amplitude and phase of the determined variation.
The step (D) calculation of the drive motor profile may comprise calculation of the drive torque variation in accordance with a sum of torques model, in that the drive torque variation comprises variation in torque due to radius variation of the first spool in the sum of torques model.
In another embodiment, the drive motor profile is calculated in an initializing operation, upon the web having substantially different diameters at each of the spools, such that the spools are rotated at substantially different rotational frequencies.
In a further embodiment, compensation is provided for web tension variation caused by both spools. Specifically, the second tachometer also has an index indication. In the method, the control system:
(1A) determines, from the first tachometer, the rotational frequency of the first spool;
(1B) determines, from the second tachometer, variation in rotational velocity at the second spool which occurs at the frequency of the (1A) determined rotational frequency of the first spool;
(1C) determines, from the first tachometer index indication, the rotational position at the first spool corresponding to the (1B) determined variation in rotational velocity at the second spool;
(1D) calculates a first drive motor profile for operating the first drive motor which tends to cancel the (1B) determined variation in rotational velocity at the second spool;
(1E) superimposes the (1D) drive motor profile on the first drive motor at the (1C) determined rotational position of the first spool;
(2A) determines, from the second tachometer, the rotational frequency of the second spool;
(2B) determines, from the first tachometer, variation in rotational velocity at the first spool which occurs at the frequency of the (2A) determined rotational frequency of the second spool;
(2C) determines, from the second tachometer index indication, the rotational position at the second spool corresponding to the (2B) determined variation in rotational velocity at the first spool;
(2D) calculates a second drive motor profile for operating the second drive motor which tends to cancel the (2B) determined variation in rotational velocity at the first spool; and
(2E) superimposes the (2D) drive motor profile on the second drive motor at the (2C) determined rotational position of the second spool.
For a fuller understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.